The prevention of hemorrhagic stroke in patients with unruptured intracranial aneurysms (UIA) may be possible with a better understanding of the natural history of these lesions. The current study forms part of the International Study of Unruptured Intracranial Aneurysms (ISUIA). Its primary objectives are: (1) to define a critical aneurysmal size above which there is a significant risk of future rupture among patients without a history of subarachnoid hemorrhage (SAH); (2) to compare the risk of future rupture of UIA, disability, and death among patients with and without a history of prior SAH from a different source and to determine whether or not the risk of future rupture varies directly with aneurysmal size among patients with a history of prior SAH; and. (3) to define the surgical morbidity and mortality involved with repair of UIA across a broad spectrum of populations and surgeons with special reference to the size and location of the aneurysm, history of SAH from another source, and other confounding variables such as age and associated medical conditions. The study will involve collecting and analyzing both retrospective and prospective data. The retrospective portion includes identification of 1,500 patients with unoperated UIA by records review (from 1970 to 1989) at participating centers. Approximately half of these patients will have no history of SAH (Group I) and the other half will have a history of SAH from a separate intracranial aneurysm that has been surgically repaired (Group II). Hard copy cerebral arteriograms will be reviewed centrally and patients will be followed by a standardized patient questionnaire with a minimum follow-up of five years from angiographic diagnosis of UIA. During the first three years of the study, 2,000 additional patients will be identified prospectively, for a two to five year prospective follow-up. Approximately 800 of these patients will be unoperated (Groups I and II) and 1,200 others will be operated. Prospectively identified patients will be followed at one month and annually until study termination. The primary analysis will examine neurologic outcome, specifically, fatal and nonfatal intracranial hemorrhagic stroke secondary to aneurysmal rupture. Secondary analyses of neurologic outcome will examine other aneurysmal complications such as ischemic stroke and death from all causes. The prognostic significance of several independent, variables with respect to stroke due to aneurysmal rupture and mortality, will also be analyzed using a Cox proportional hazards model.